Sunday, June 7, 2009

Do"s and Dont"s of a Thunderstorm Flying

Flight EnvironmentTHUNDERSTORM HAZARDS

The dangers of flying in or close to a thunderstorm are:

1. Turbulence. Turbulence, associated with thunderstorms, can be extremely hazardous, having the potential to cause overstressing of the aircraft or loss of control. Thunderstorm vertical currents may be strong enough to displace an aircraft up or down vertically as much as 2000 to 6000 feet. The greatest turbulence occurs in the vicinity of adjacent rising and descending drafts. Gust loads can be severe enough to stall an aircraft flying at rough air (maneuvering) speed or to cripple it at design cruising speed. Maximum turbulence usually occurs near the mid-level of the storm, between 12,000 and 20,000 feet and is most severe in clouds of the greatest vertical development.Severe turbulence is present not just within the cloud. It can be expected up to 20 miles from severe thunderstorms and will be greater downwind than into wind. Severe turbulence and strong out-flowing winds may also be present beneath a thunderstorm. Microbursts can be especially hazardous because of the severe wind shear associated with them.2. Lightning. Static electricity may build up in the airframe, interfering with operation of the radio and affecting the behaviour of the compass. Trailing antennas should be wound in. Lightning blindness. may affect the crew's vision for 30 to 50 seconds at a time, making instrument reading impossible during that brief period. Lightning strikes of aircraft are not uncommon. The probability of a lightning strike is greatest when the temperature is between -5ºC and 5°C. If the airplane is in close proximity to a thunderstorm, a lightning strike can happen even though the aircraft is flying in clear air. Lightning strikes pose special hazards. Structural damage is possible. The solid state circuitry of modem avionics is particularly vulnerable to lightning strikes. Electrical circuits may be disrupted. The possibility of lightning igniting the fuel vapor in the fuel cells is also considered a potential hazard.3. Hail. Hailstones are capable of inflicting serious damage to an airplane. Hail is encountered at levels between 10 and 30 thousand feet. It is, on occasion, also encountered in clear air outside the cloud as it is thrown upward and outward by especially active cells.

4. Icing. Heaviest icing conditions occur above the freezing level where the water droplets are supercooled. Icing is most severe during the mature stage of the thunderstorm.

5. Pressure. Rapid changes in barometric pressure associated with the storm cause altimeter readings to become very unreliable.

6. Wind. Abrupt changes in wind speed and direction advance of a thunderstorm present a hazard during take-off and landing. Gusts in excess of 80 knots have been observed.Very violent thunderstorms draw air into their cloud bases with great intensity. Sometimes the rising air forms an extremely concentrated vortex from the surface of the ground well into the cloud with vortex speeds of 200 knots or more and very low pressure in its center. Such a vortex is known as a tornado.

7. Rain. The thunderstorm contains vast amounts of liquid water droplets suspended or carried aloft by the updrafts. This water can be as damaging as hail to an aircraft penetrating the thunderstorm at high speed. The heavy rain showers associated with thunderstorms encountered during approach and landing can reduce visibility and cause retraction on the windscreen of the aircraft, producing an illusion that the runway threshold is lower than it actually is. Water lying on the runway can cause hydroplaning which destroys the braking action needed to bring the aircraft to a stop within the confines of the airport runway. Hydroplaning can also lead to loss of control during take-off.

ST. ELMO'S FIREIf an airplane flies through clouds in which positive charges have been separated from negative charges, it may pick up some of the cloud's overload of positive charges. Weird flames may appear along the wings and around the propeller tips. These are called St. Elmo's Fire. They are awe-inspiring but harmless. It the airplane flies in the vicinity of a cloud where negative charges are concentrated, its positive overload may discharge into the cloud. In this case, it is the airplane which strikes the cloud with lightning! The electricity discharges cause a noisy disturbance in the lower frequency radio bands but do not interfere with the very high frequencies. This precipitation static, as it is called, tends to be most severe near the freezing level and where turbulence and up and down drafts occur.

THUNDERSTORM AVOIDANCE

Because of the severe hazards enumerated above, attempting to penetrate a thunderstorm is asking for trouble. In the case of flight, airplane pilots, the best advice on how to fly through a thunderstorm is summed up in one word—DON'T.Detour around storms as early as possible when encountering them enroute. Stay at least 5 miles away from a thunderstorm with large overhanging areas because of the danger of encountering hail. Stay even farther away from a thunderstorm identified as very severe as turbulence may be encountered as much as 15 or more nautical miles away. Vivid and frequent lightning indicates the probability of a severe thunderstorm. Any thunderstorm with tops at 35,000 feet or higher should be regarded as extremely hazardous. Avoid landing or taking off at any airport in close proximity to an approaching thunderstorm or squall line.Microbursts occur from cell activity and are especially hazardous if encountered during landing or take-off since severe wind shear is associated with microburst activity. Dry microbursts can sometimes be detected by a ring of dust on the surface. Virga falling and evaporating from high based storms can cause violent downdrafts.

The gust front, another zone of hazardous wind shear, can be identified by a line of dust and debris blowing along the earth's surface.Swirls of dust or ragged clouds hanging from the base of the storm can indicate tornado activity. If one tornado is seen, expect others since they tend to occur in groups.Do not fly under a thunderstorm even if you can see through to the other side, since turbulence may be severe. Especially, do not attempt to fly underneath a thunderstorm formed by orographic lift. The wind flow that is responsible for the formation of the thunderstorm is likely to create dangerous up and down drafts and turbulence between the mountain peaks.Reduce airspeed to maneuvering speed when in the vicinity of a thunderstorm or at the first indication of turbulence.

Do not fly into a cloud mass containing scattered embedded thunderstorms unless you have airborne radar.

Do not attempt to go through a narrow clear space between two thunderstorms. The turbulence there may be more severe than through the storms themselves. If the clear space is several miles in width, however, it may be safe to attempt to fly through the center, but always go through at the highest possible altitude. When flying around a thunderstorm, it is better to fly around the right side of it. The wind circulates anti-clockwise and you will get more favorable winds. If circumstances are such that you must penetrate a thunderstorm, the following few simple rules may help you to survive the ordeal:

1.~ Go straight through a front, not across it, so that you will get through the storm in the minimum amount of time.

2.~ Hold a reasonably constant heading that will get you through the storm cell in the shortest possible time.

3.~ Before entering the storm, reduce the airspeed to the airplane's maneuvering airspeed to minimize structural stresses.

7.~ Determine the freezing level and avoid the icing zone. Avoid dark areas of the cell and, at night, those areas of heavy lightning.

8.~ Do not use the autopilot. It is a constant altitude device and will dive the airplane to compensate for updrafts, causing excessive airspeed, or will cause the plane to climb in a downdraft creating the risk of a stall.